The present invention relates to machines for shaping single crystal ingots, and in particular relates to a chuck for holding a conical end of such an ingot while the ingot is being rotated and machined to a desired shape.
Semiconductor substrates to be used in semiconductor integrated circuits are manufactured by centering a single crystal ingot, produced in the general shape of a round bar with tapered ends by a single crystal growing method such as the Czochralski method, grinding the generally cylindrical part of the ingot with a grinder to finish it with a prescribed diameter, and thereafter cutting the shaped ingot perpendicular to its longitudinal axis to obtain generally planar wafers, which are then further prepared to produce integrated circuits.
A single crystal ingot has generally conical end parts that are centered in a lathe, using a chuck located at each end thereof to grip the generally conical end parts so that the ingot can be ground to a required shape. In the past, chucks defining concave conical socket surfaces have been used at the headstock and the tailstock of a lathe, to hold the conical ends of a single crystal ingot.
The generally conical parts at the ends of a single crystal ingot, as a result of the process of their formation, have somewhat irregularly shaped surfaces including protruding nodes. The end portions of an ingot thus contact the interior surfaces of a conical socket portion of a lathe chuck with irregularly concentrated pressure.
Axial pressure is exerted between the tailstock and headstock of a lathe in supporting and rotating a single crystal ingot, as is explained, for example, in Hirano et al. U.S. Pat. No. 5,525,092, and such pressure brings the surfaces of the conical end portions of such a single crystal ingot against the surfaces of the chucks used to hold such an ingot in a lathe. Pressure concentrations caused by nodes in the conical end portions of such a single crystal ingot are thus imposed on the surfaces of such chucks.
The previously known chucks are of unitary construction and have axially oriented bolt holes that intersect their conical interior socket surfaces. An ingot held in such a chuck will typically slip until a node on its surface engages an open bolt hole and interlocks with it. This stops the slippage, but may often result in the ingot being located eccentrically. As machining of the ingot progresses the ingot later may slip into a different position in the chuck.
Also, because of the irregular surfaces of the conical end portions of the ingots, the surfaces of the interior of the previously used conical chucks are soon damaged, and, particularly at the margins of the holes through which bolts are inserted to fasten such chucks to the headstock or tailstock of the lathe, the deformation of the material surrounding the holes makes the chucks unsatisfactory for further use undesirably quickly, as deformities surrounding bolt holes become more likely to engage nodes.
It is necessary to center the conical ends of such single crystal ingots and to grip them securely so that the ingots can be rotated without excessive slipping and so that they can be shaped accurately into the required generally cylindrical shape in preparation to being cut into planar wafers. Using previously available chucks it has been difficult both to center an ingot accurately and to hold it securely enough to rotate it in a controlled manner so that it can be shaped as required without rapidly damaging the chuck surfaces contacted by the ingots.
When a damaged chuck is replaced it must be mounted precisely centered on the grinder lathe, or unacceptable vibration will result when ingots are rotated. The process of mounting such chucks thus takes significant amounts of time.
What is desired, then, is an improved chuck for holding an end of an object such as a single crystal ingot of a semiconductor material securely in a centered position and for transferring ample forces to such an object to rotate it as it is machined or otherwise shaped as required.